Thermostatic control switch



1969 c. R. TURNER ET AL 3,425,016

THERMOSTATIC CONTROL SWITCH INVENTORSI CHARLES R. TURNER WALTER E. MYERS LUTHER P. MANSHIP WW ATT YS-.

Filed Oct. 27, 1966 United States Patent O 3,425,016 THERMOSTATIC CONTROL SWITCH Charles R. Turner and Walter E. Meyers, Philadelphia,

and Luther P. Manship, Collingdale, Pa., assignors to Proctor-Silex Incorporated, Philadelphia, Pa., a corporation of New York Filed Oct. 27, 1966, Ser. No. 589,88

U.S. Cl. 337--1 14 Claims Int. Cl. H01h 37/46 The present invention relates to improvements in a thermostatic control switch used to control temperature or some other heat related effect in an associated electric appliance or electrical device. More specifically, the present invention concerns temperature responsive switches wherein response to resistance heating effects within the switch is improved to the end that controlled thermal effects can be regulated with greater accuracy.

Thermostatic control switches such as the switch shown in United States Patent 3,282,196 of Manship et al. have been subject to variations in the conditions under which the switch contacts open and close. This has resulted in serious discrepancies in the performance from one time to another of electrical appliances under the control of such switch. Since reproducibility of performance and accuracy within a certain degree of tolerance is a normal requirement, such switches, because of the multiplicity of factors effecting them, have frequently failed to meet specifications.

An analysis of the problem involved indicates that there are many variables which affect switch performance, including as lack of consistent pressure between the contacts, changeable ambient temperature in the region of the switch, non-uniform time required during successive cycles of operation for bimetallic elements to flex, and non-uniform and non-repetitive flexing characteristics in bimetallic elements, particularly those heated by their own internal resistance to current flow. These and other effects have contributed to erratic local heating effects in the region of the switch contacts which have been proved to be a major obstacle in repetivity of performance. These local heating effects, by their very nature, are non-repetitive and variable from time to time depending upon a multiplicity of factors. Local heating in itself has caused the contacts to separate at widely different temperatures of the heated region whose temperature is to be controlled when the switch has been set to respond to a single specific temperature.

In accordance with the present invention, a switch construction is provided in which uniform, repetitive switch response can be obtained. In particular, the present invention takes advantage of the local effect at the contacts to accelerate the opening of the switch contacts. This can be done because it is characteristic that the heating effects at the contacts occur primarily as the contacts are about to open so that by using whatever heating occurs to cause immediate opening of the contacts, the variable factors produced by such local heating and other factors in combination with the local heating are eliminated. Furthermore, in accordance with this invention the construction of a bimetallic element heated primarily by its internal resistance to current flow is such that its performance is repetitive.

In accordance with the present invention a switch is provided which will open or close at a selected control point. Moreover, the switch is capable of use in which its effect is extending the cycle length during which the switch is closed and extending the time during which the switch contacts are open.

The present invention is directed to a condition responsive device which opens at a desired condition. This device constitutes a pair of normally closed switch contacts positioned relative to one another so that they may be opened, a mechanical support for one of the contacts serves also as an electrical connection and includes a single bimetal strut of general channel shape. The web area is cut away between the side flanges over a portion of the length of the channel, leaving a narrow bridge at the contact support end supporting one of the contacts midway between the side flanges. This bridge constitutes a secondary thermomotive zone rapidly responsive to thermal effects near the contacts to end to move the supported contact in one direction. Separated from the secondary zone by the cut away web area is a primary zone provided by a web area. The side flanges are interrupted along the length of the bimetal strut at the primary zone to permit the bimet-al to flex lengthwise to produce an opposite effect upon the contact from the secondary zone. The thermal effect in response to heat generated by internal resistance to flow of current through the strut is normally greater in the primary than in the secondary zone. However, the thermal effect in response to heat generated at said switch contacts is initially greater in the secondary than in the primary zone.

For a better understanding of the invention, reference is made to the following drawings in which:

FIG. 1 is a side elevational view partially in section of a coffee percolator employing a thermostatic control switch in accordance with the present invention;

FIG. 2 is an enlarged side elevational view of the switch shown in FIG. 1 partially in section to show details of switch construction and having its terminals shown connected to a schematic circuit including the primary and secondary heater elements of the coffee percolator;

FIG. 3 is a perspective view partially broken away to show the construction of a bimetallic strut in accordance with the present invention carrying one of the switch contacts;

FIG. 4 is a side elevational view partially sectioned showing the bimetallic strut of FIG. 3 in operation in closed contact position; and

FIG. 5 is a view taken along line 55 of FIG. 4.

The thermosetatic control switch of the present invention finds particular use in controlling the operation of an electric percolator, such as seen in FIG. 1, generally designated 10. The thermostatic control switch 40 is supported within the base 12 of the percolator. Except for the thermostatic control switch 40, the percolator shown in FIG. 1 is similar to the percolator shown and described in United States Patent 3,282,196 of Manship et al., and of common assignee herewith. This percolator is used by way of illustration and it will be understood by those skilled in the art that a percolator of another construction, or other electrical appliances and devices, may also employ the thermostatic control switch of the present invention.

Briefly, the percolator shown in FIG. 1 comprises a generally cylindrical, hollow cup-shaped base 12, a separate handle 14 secured to the base and extending upwardly from one side thereof, and a container 16 nested within the lip of base 12 and clamped on the base by latch means generally designated 18 in the upper portion of handle 14. The container is shown surmounted with a sheet metal lip 20 having a knob 22, the lid enclosing the top portion of a coffee basket 24 supported by a fountain tube 26 extending downwardly from the coffee basket to the base 12. A cup-shaped heat-pump well 28 is centrally positioned in the base. The upper lid of the heat-pump well provides a collar 28a which extends upwardly into container 16 through an aperture 16a centrally located in the bottom of the container. Two resistance heaters 30 and 32 are incorporated in and around the sidewalls of heat-pump well 28 and are electrically insulated therefrom but in heattransfer relationship with the well. Primary heater 30 is adapted to perform the percolating operation, and secondary heater 32 is provided to maintain the brewed coffee at a predetermined temperature after percolation is completed. A transluscent flexible disc 34 has a central opening which is positioned to permit disc 34 to slip over the upper end of collar 28a around well 28, the disc 34 providing a seal between the well and the sidewalls of the base in leak-tight relationship there'between.

The fountain tube 26 is provided with a cup-shaped disc 26a secured near the lower end of the tube, the disc being of such diameter that it will pass through the upper portion of collar 28a into well 28 and seat against a beveled surface along the Wall of the well. Disc 26a is provided with apertures in a conventional manner to permit liquid from upper chamber 28b of the heat-pump well 28 to enter lower chamber 28c of said well 28, the fountain tube disc 26a dividing the well into the upper and lower chambers. Also, a conventional circular valve disc 26b is loosely mounted beneath disc 26a to close the apertures when pressure is developed within lower chamber 28c of the heat-pump well 28. Pressure in lower chamber 280 causes heated water to be driven up fountain tube 26 and overflow the top thereof into coffee basket 24. Repetition of this process with successive increments of water repeats until coffee is brewed in accordance with temperature control by a switch device to be described below. For a more detailed description of the percolators structure, reference is had to the United States Patent 3,282,196 of Manship et al., above identified.

The thermostatic control switch shown in the abovecited United States Patent 3,282,196 of Manship et al. has various components which function similarly to certain components in the present invention. The thermostatic switch and related circuit described in that United States patent has many advantages as pointed out therein, but that system has limitations which the present invention is designed to overcome. In the structure of the present invention beneath flange 36 of well 28 a horizontally extending bracket 38 is attached by brazing or welding so that a good conductive heat flow path is provided. Bracket 38, in turn, supports the thermostatic switch assembly, generally designated 40, all of the operative parts of which are held together by a bolt 42 and nut 44, as shown in FIGS. 1 and 2. The thermostatic control switch comprises a bimetallic member 46 which is clamped tightly against bracket 38 and, therefore, is in good thermal communication with the heat well 28 through its flange 36 and bracket 38. Switch blades and a bracket 48 are stacked beneath bimetallic member 46 and are separated from the bimetallic member and from each other by similar annular electrical insulators 50. The bolt 42 which holds the stacked pieces together passes through openings in the switch blades large enough and positioned to avoid electrical contact between the bolt and the blades. The switch blades include a first resilient conductive switch blade 52 carrying a contact 54; a second resilient conductive switch blade 56 carrying contacts 58 and 60 on em posite faces positioned to engage, respectively contact '54 and contact 62. Contact 62 is supported by a third resilient conductive switch blade generally designated 64. As seen in FIG. 2 contacts 60 and 62 through their respective support blades are connected in series with and control the primary heater 30 whereas contacts 54 and 58 through their respective support blades are connected in series with and control secondary heater 32.

As more clearly seen in FIG. 3 switch blade 64 cornprises a short bimetallic strut 66, supported on a resilient strut 68. Strut 68 has a portion 68a which is supported in the stack and a web portion 68b which is essentially rigid because of channel flanges 68c extending along both edges over its length. The center of portion 68b is cut away over most of the width between side flanges 68c. Strut 66 is secured at one end under strut 68 and strut 66 extends beneath the opening in strut portion 68b. The opening is wider and extends further than strut 66. At its free end strut 66 carries contact 62 on its top surface in position to contact contact 60 (see FIG. 2) through the opening in strut portion 68b, as shown in FIGS. 2 and 4.

The free end of strut 68 projecting beyond contact 62 supports an insulating post 72 which extends upwardly to be engageable by bimetallic member 46. The insulating post 72 has an under-cut providing a shoulder 72a overlying the extended free end of blade 56 which under certain conditions provides a stop to limit the approach of contact 58 toward contact 54. Blade 52 does not extend as far as post 72. Blade 56 is arranged to be engaged by shoulder 72a when bimetallic member 46 is bowed downwardly pressing the post 72, and hence blade 64, downward. Rigid bracket 48 beneath the switch blades has an aperture adjacent its free end which receives an adjustable fitting 74 threaded therein. The adjustable fitting has a threaded aperture to receive adjusting screw 76. Adjusting screw 76 supports, and facilitates axial adjustment of, an insulating rod 78 which projects upwardly through apertures in blades 56 and 64 and engages blade 52. Secured to the fitting 74 is an adjustment arm 80 biased downwardly by coil spring 81, which extends between arm 80 and bracket 48. The other end of arm 80 has a downwardly extending pin 82 secured thereto which functions as a cam follower in cooperation with a cam slot '84 in which it rides in lever arm 86. One end of lever arm 86 terminates in an annular portion which is supported on, encircles and rotates about the lower portion of heat-pump well 28 and the other end of the lever arm extends through an opening (not shown) in the base to be movable from outside the base. Manual movement of lever arm '86 from outside the base causes adjustment arm 80 to rotate and thereby rotates adjustable fitting 74. Since adjustable fitting 74 is threaded to an aperture in bracket 48, such rotary movement causes rod 78 to move up or down with fitting 74, depending upon the direction of rotation, and the engagement between the end of rod 78 and blade 52 causes contact 54 to respond to this vertical movement of its supporting member. Blade 56 is biased in such a manner as to urge contact 58 into contact 54 so that the two contacts are normally in engagement and overclosed, i.e., subject to more pressure than required for just closing the contacts. Therefore, adjustment of the position of blade 52 in this manner does not separate these contacts. Strut 66 is biased toward blade 56 so that contacts 60 and 62 are normally in engagement. However, when blade 52 is adjusted upward, the resultant upward movement of blade 56 tends to reduce the pressure of overclosure between contacts 60 and 62, and therefore, serves as an adjusting means to regulate the temperature at which these contacts open, as will be described hereafter.

FIG. 2 also shows a schematic circuit diagram illustrating one circuit arrangement whereby resistance heaters 30 and 32 are connected to power through switch 40. Power supply terminals are shown schematically as terminals 90 and 92. The primary heater 30 is connected at one end to terminal 98 and at the other end to blade 64. Terminal 92 is connected to blade 56. One side of secondary heater 32 is connected to blade 56. One side of secondary heater 32 is connected to blade 64 like the primary heater 30. The other side of heater 92 is connected to switch blade 52.

Since the contacts are all initially biased into engagement with each other, the primary heater is energized when the percolator is plugged in, the circuit extending from terminal 90 through primary heater 30 contacts 62 and 60 to terminal 92. Secondary heater 102 is initially effectively short circuited.

Heat from primary heater 30 tends to vaporize liquid in lower chamber 280 and thereby cause percolation from lower chamber 280 upward through the fountain tube 26 as described above. However, the liquid in upper chamber 28b is substantially at the temperature of the liquid in container 16, since it is in unrestricted conductive com munication with the liquid in the container. The thermostatic control switch is attached to well 28 at a point adjacent the upper chamber 28b. Therefore, bimetallic member 46 is in a position to sense the temperature of the liquid in upper chamber 28b, which is representative of the body of liquid in container 16. As the temperature rises, the free end of bimetallic member 46 moves downwardly and presses on post 72 which moves blade 64 downwardly until contact 62 separates from contact 60. The temperature at which this occurs is determined by the setting of adjustable fitting 74 and adjusting screw 76. The opening of contacts 60 and 62 opens the circuit of primary heater 30 and therefore stops the percolation process. Upon the opening of contacts 60 and 62 the short circuit across secondary heater is removed and the secondary heater 32 becomes energized to keep the brew warm. The circuit is completed from terminal 90 through primary heater 30 which is energized at a much lower level, secondary heater 32, and contacts 58 and 54 to terminal 92. The combined resistance of heaters 32 and 30 reduces the current flow in the circuit so that the heat ing eflect on the liquid is maintained at a level which merely keeps the brew Warm.

As previously stated, bimetallic strut 66 which supports contact 62 at its free end is supported by strut 68, which is, in turn, supported by annular insulating means 50. As shown in FIGS. 3 and 4, strut 68 has channel flanges 680 along its length to prevent flexing of the strut along the length of the flanges. Consequently, as post 72 supported at the free end of the strut 68 is moved downwardly by flexing of bimetallic member 46, strut 68 will not flex in the channel area of flange 680. Any flexing of strut 68 is confined to the area between its support by insulation 50 and the point at which the flanges 68c begin. Bimetallic blade 66 is supported by strut 68 in the aperture 70 between the flanges 680 in such manner that bimetallic blade 66 may flex relative to strut 68 upon heating and move in the aperture 70 of strut 68.

The shape of the bimetallic strut 66 is best seen in FIG. 3. Strut 66 is a generally channel shaped bimetal having a primary flexing portion 66a and a secondary flexing portion 66b.

The channel form of strut 66 is considerably modified by portions which are cut out of the bimetal. These areas can be punched out before folding the side flanges 660. The flanges themselves extend almost the full length of the strut but are interrupted at the primary zone 66a in order to permit heating to produce flexing in that area. By location of the gap in the side flanges the location of the effective part of the primary zone 6612 can be adjusted to provide a desired lever arm distance from contact 62. The primary zone 66a is a planar U-shaped region but only a portion of the legs of the U extending in the lengthwise direction are eflective. Since their overall cross-sectional area in this region is much reduced over other parts of the strut the current density in this area is greatly increased thereby increasing the heating effect which, of course, increases with the square of current. By placing the interruption of the side flanges at the area where it is desired to have flexing occur, the flexing in response to internal resistance to current flow in the primary zone always occurs in the same way in the same region and the effect is repetitive each time it occurs. The web is completely cut away in the region between the primary and secondary zones leaving only the side flanges to interconnect them. The secondary zone 66b is a very narrow bridge with current flowing from the ends to center, or vice versa, and the heating effect is heightened by the small sectional area of the bridge across the direction of current flow. The fact that the two zones are part of the same bimetal eliminates possible variations which might occur if two separate bimetals were employed having, for instance, differences in the index of flexure or differences in thickness. Because the two zones are part of the same bimetal, many errors which might arise because of the use of two bimetals are self-compensating or selfcorrecting.

As bimetallic strut 66 is heated the primary portion 66a begins to flex along the legs of the U in the region of the gap in the side flanges to move contact 62 toward contact 60, as shown in FIG. 4. At the same time, the secondary portion 66b flexes, as shown in FIG. 5, between the side flanges and tends to move contact 62 away from contact 60. The primary zone is dominated because of the effect of the longer lever arm over which it acts. The primary zone maintains contact 62 in contact with contact 60 until blade 64 through action on strut 68 has been moved by post 72 by the action of thermomotive member 46 a sufficient distance almost to separate the contacts. At or near this point the reduction in contact pressure causes an increase in electrical resistance and/ or arcing between the contacts which, in turn, causes increased local heating. The local heating effect causes rapid heating of the secondary zone causing the contact 62 to be moved rapidly away from contact 60 causing the contacts to separate.

Bimetallic strut 66, which supports contact 62, is heated by passage of current therethrough when contacts 62 and 60 are in engagement. Bimetallic strut 66 is so oriented that, when it cools as a result of separation of contacts 62 and 60, the secondary thermomotive zone will tend to move contact 62 in a direction to decrease the gap between the contacts but the primary thermomotive zone will move contact 62 a sufliciently greater distance in the opposite direction to increase the gap between the contacts. When the percolator has been disconnected for serving coffee or for any other purpose and is again plugged in, the secondary heater 32 will be reenergized, but the action of the primary portion of strut 66 will prevent repercolation unless the time interval has been sufliciently long that the temperature of the brew has dropped below the temperature at which thermomotive member 46 acting through post 72 will hold contact 62 away from contact 60, or approximately 30 This arrangement prevents undesirable recycling of already brewed coffee through spent grounds.

In the prior art other heating effects, in addition to the current flow, have been found to cause variations in the operation of thermomotive members. For example, varia tions in heating are the result of local heat generated at the contacts as a bimetallic strut starts to cause separation of the contacts. Initially the heating is local in the immediate region of the contacts. In the prior art switches such as seen in United States Patent 3,282,196 such factors as pressure between the contacts, ambient temperature in the percolator base, and the time required during successive cycles of operation for the bimetal corresponding to member 46 to flex and move blade 68 to open the contacts have all contributed to instability. In addition, slow flexing of the bimetal corresponding to member 46 and movement of the bimetallic blade corresponding to blade 66 has caused sparking between the contacts corresponding to contacts 60 and 62 thus producing further heat transmitted to the bimetallic blade which is of a random unpredictable amount. These variable heat effects have been inconsistent for each opening of the contacts, so that heating effects cannot be accurately taken into account in calibrating the control switch to open at a desired temperature of the brew for each cycle of operation.

The prior art bimetallic switch blade contributes to the indefiniteness of the opening of the thermostatic switch so that a desired range of temperatures of the brew in the container cannot be accurately maintained. The variable area of flexing of such bimetallic switch blades, usually flexing over portions of their entire length, causes variation in the opening of the switch on successive cycles of operation. It was found by measurement of many prior art control switches as employed in a percolator that variations in the temperature at which heating of the brew was terminated on successive cycles of operation varied over a range of up to 18 F.

In operation of the modified bimetal 66 of the present invention as the primary zone 66a is heated, the primary zone elements flex as seen in FIG. 4 and the pressure between contacts 60 and 62 is increased. At the same time secondary zone 66b opposes this action but is normally too small to be effective. However, as thermomotive member 46 moves downward under increasing temperatures of the fluid in heat well 28, contacts 60* and 62 reaching the point of separation experience a startling rapid heating due to the resistance increase which produces momentary movement by the secondary zone in response to the local heating at the contacts so that the secondary zone momentarily dominates the primary zone tending to open the contacts. If sparking occurs it tends to increase this effect. However, ordinarily the separation of contacts causes the normal resistance heating to stop so that the action of the primary zone begins at once to withdraw contact 62 from 60 and therefore accelerate opening of the switch. The present bimetallic blade 66 eliminates the problems caused by variable heats by the action of secondary flexing zone.

The secondary zone in acting rapidly eliminates most of the variable heating effects. As these variable heating effects cause the primary zone to flex upwardly closing contacts 60 and 62, the secondary zone flexes to open the contacts compensating and balancing the variable heating effects. The design of the bimetallic strut 66 is such that two definite areas of flexing are provided by the primary and secondary zones and are isolated from one another sufficiently to facilitate consistency of flexing for accurate opening of the switch at a predetermined calibrated temperature. When the bimetal 46 begins to open the contacts instead of delay and hesitancy, the secondary zone immediately effects opening due to the pronounced effect on secondary zone 66b. :It is important for uniformity of coffee strength on repetitive cycles of operation of the percolator that the temperature reached in each cycle of operation be consistent. Upon repeated testing of the thermostatic control switch of the present invention in the percolator described, the variations in the temperature at which heating of the brew was terminated on successive cycles were consistently within only a few degrees of the calibrated temperature for opening of the switch.

Should there be no liquid in container 16 to absorb the heat, the well 28 will rise in temperature which will directly effect bimetal member 46 causing it to bow down wardly. This will promptly open contacts 62 and 60, and further bowing will cause shoulder 72a of insulator post 72 to engage the free end of blade 56 to separate contacts 54 and 58. In this manner both heaters are deenergized before any damaging temperature is reached. In the arrangement described, a single thermostatic switch is directly responsive to the temperature of the liquid in the container and additionally serve as a disconnecting switch to guard against overheating.

It -will be observed that in accordance with the present invention an improved thermostatic switch blade is provided for accurate operation of a thermostatic control switch for regulating flow of electrical current in response to temperature variations. The new bimetallic flexible switch blade of the present invention having a primary thermomotive zone and a secondary thermomotive zone isolated from the primary zone will give accurate and consistent opening of the thermostatic control switch at a desired predetermined temperature. The secondary zone of the bimetallic blade compensates for variable heat effects which have caused irregular operation of prior art control switches and, hence, the devices in which the control switches were employed. It will be appreciated by those skilled in the art that the thermostatic control switch of the present invention fulfills the need for an accurate means of regulating the heating of an electrical device to provide accurate thermostatic operation.

While the invention has been described with particular reference to a specific embodiment thereof in the interest of complete definiteness, it should be understood that it may be embodied in a large variety of forms diverse from the ones specifically shown and described, without departing from the scope and spirit of the invention as defined by the appended claims.

We claim:

1. A condition responsive device to open at a desired condition comprising: a pair of switch contacts positioned relative to one another so that they may be opened and closed, a mechanical support for one of the contacts serving also as an electric-a1 connection and including a single bimetal strut of general channel shape having web area cut away between the side flanges thus separating we'b areas constituting a primary and a secondary zone and leaving a narrow bridge at the contact supporting end supporting one of the contacts at a point between the side flanges, said bridge constituting said secondary thermomotive zone and being rapidly responsive to thermal effects near the contacts to tend to move the supported contact in one direction, and having the side flanges interrupted along its length to provide a web area primary zone of opposite effect upon the contact from the secondary zone, the thermal effect in response to heat generated by internal resistance to flow of current through the strut being greater in the primary than in the secondary zone but the thermal effect in response to heat generated at said switch contacts being initially greater in the secondary than in the primary zone.

2. The thermostatic control switch of claim 1 in which the middle of the web is cut away at the primary zone to reduce the cross-sectional area in the primary zone and to leave a pair of generally parallel longitudinally extending legs in the region where the side flanges are interrupted.

3. The thermostatic control switch of claim 2 in which the primary zone tends to close the contacts and the secondary zone tends to open the contacts and a further thermomotive element provides the major effect tending to open the controls.

4. A normally closed thermostatic switch means for use with a heated region whose temperature is to be controlled comprising: a pair of normally closed switch contacts positioned relative to one another so that they may be moved open and closed, a support blade including at least in part a bimetallic strut formed to provide primary and secondary zones which act to move the supported contact in opposite directions from each other, the primary zone acting to tend to further close the contacts, being further removed from the supported contact and therefore providing a longer and more effective lever arm than the secondary zone and the secondary zone being in substantially closer proximity to the contacts than the primary zone so that the secondary zone is immediately effected by local heating effects at the contacts and will respond to such an effect in such a manner as momentarily effectively to dominate contact movement and thermomotive means in conductive contact with the heated region and acting to open the contacts against the action of the primary zone when the heated region temperature reaches a predetermined level.

5. The switch of claim 4 in which the contacts are supported by a pair of mutually insulated current conducting flexible blades and the thermomotive means in conductive contact with the heated region is a third blade mutually insulated from the other two.

6. The switch of claim 5 in which the bimetallic strut provides the support blade for one of the contacts is of general channel shape having web area cut away between the side flanges leaving a narrow bridge at the contact end supporting one of the contacts midway between the side flanges and said bridge constituting the secondary thermomotive zone and having the side flanges interrupted along its length to provide the primary zone.

7. The switch of claim 6 in combination with a coffee maker having a heat well within a fluid container, said heat well carrying a first electric heater in electrical con nection with the switch contacts and a source of power and in heat transfer relationship with the heat well for heating fluid within the container and the thermomotive means in contact with the heated means is mounted in conductive contact with said beat well.

8. The switch of claim 7 in which there is provided a warming electric heater also in heat transfer relationship with the well to keep the fluid in the coffee maker warm and a second pair of contacts connected to the warming heater and a source of power, the thermomotive means in contact with the heat well is a bimetal blade and means provided in association with movement of the bimetal blade to open the second pair of contacts upon the assumption of a predetermined posit-ion by the third blade.

9. The switch of claim 8 in which contacts are carried on flexible blades mutually insulated from one another except for the contacts, said blades, including the thermomotive means in heat flow contact with the heat well, being stacked and a post acting between the unsupported ends of the thermomotive means and the bimetal strut carrying one of the contacts and attached to one of them acting to separate sequentially the first and second pairs of contacts as the temperature of the thermomotive means rises.

10. A thermostatic switch comprising two pairs of contacts each supported on at least three flexible struts mutually insulated from one another so that no more than one contact of each pair is carried on a strut, one of the end struts including a bimetallic blade having two zones, a narrow secondary zone supporting and being in good thermal contact with the supported contact and acting upon being heated to separate the contacts, and a primary ZOne thermally separated from the secondary zone and acting to tend to further close the contacts and a further bimetallic strut in the stack at the opposite end from the bimetallic blade and insulated from the other struts being in heat conducting relationship with a heated region whose temperature is to be controlled and responding to increasing temperatures to flex toward said bimetallic blade and an insulating element interposed between the further bimetallic str-ut and the bimetallic blade and fixed to one of them to cause the cont-act supported by the bimetallic blade to be opened.

11. The thermostatic switch of claim 10 in which the insulating element is fixed to a strip to which the bimetallic blade is aflixed and said insulating element is provided with a shoulder to engage another of the blades to open the second pair of cont acts in response to further heating of the further bimetallic strut after the opening of the first pair.

12. The thermostatic switch of claim 10 in which the blades supporting the contacts are biased into a position holding the contacts closed in overclosed position by the end blade closest to the further bimetallic strut, which end blade carries one contact of the pair arranged to open later and calibration means acting on said end blade is provided for partially relieving the bias overclosing the contacts to change the point at which said contacts will open.

13. The thermostatic switch of claim 11 in which the blade supplying the bias relieved is closest to the further bimetallic strut and the bimetallic blade carrying a contact of the first pair is the most remote from the further bimetallic strut.

14. The thermostatic switch of claim 13 in which the calibration means is adjustable by means calibrating the setting to a desired temperature and further means selects different temperatures of the heated region whose temperature is to be controlled at which said first pair of contacts to open shall open.

References Cited UNITED STATES PATENTS 9/1939 Sardeson 200-113 11/1966 Manship et al 294-29 US. 01. X.R. 

1. A CONDITION RESPONSIVE DEVICE TO OPEN AT A DESIRED CONDITION COMPRISING: A PAIR OF SWITCH CONTACTS POSITIONED RELATIVE TO ONE ANOTHER SO THAT THEY MAY BE OPENED AND CLOSED, A MECHANICAL SUPPORT FOR ONE OF THE CONTACTS SERVING ALSO AS AN ELECTRICAL CONNECTION AND INCLUDING A SINGLE BIMETAL STRUT OF GENERAL CHANNEL SHAPE HAVING WEB AREA CUT AWAY BETWEEN THE SIDE FLANGES THUS SEPARATING WEB AREAS CONSTITUTING A PRIMARY AND A SECONDARY ZONE AND LEAVING A NARROW BRIDGE AT THE CONTACT SUPPORTING END SUPPORTING ONE OF THE CONTACTS AT A POINT BETWEEN THE SIDE FLANGES, SAID BRIDGE CONSTITUTING SAID SECONDARD THERMOMOTIVE ZONE AND BEING RAPIDLY RESPONSIVE TO THERMAL EFFECTS NEAR THE CONTACTS TO TEND TO MOVE THE SUPPORTED CONTACT IN ONE DIRECTION, AND HAVING THE SIDE FLANGES INTER- 